We intend to acquire more detailed information on the actual redox states of the Mn-cluster of Photosystem II, in the different S-states, by way of Mn K-edge XANES. The two sets of XANES data on flash-induced changes in the OEC presently available (performed by two different groups, on two different kinds of preparations) do not agree with each other in some essential aspects of the involvement of Mn oxidation in the various S-state transitions. It is our specific aim to perform XANES experiments on the different types of samples, but under otherwise identical conditions, by a collaborative effort with the other group. This should enable us to settle the existing discrepancy on the involvement of Mn-redox chemistry in the different S-state transitions, which will be essential for understanding the mechanism of photosynthetic water oxidation.